1. Field of the Invention:
This invention relates to a method of producing a metallic member having a unidirectionally solidified structure and in the form of a wire, bar or sheet, or of any other desired cross sectional shape. More particularly, it is concerned with a method which produces a metallic member having a unidirectionally solidified structure by passing through a heating mold having a melting zone a metallic material prepared by casting or plastic working and having a length which is considerably greater than its diameter or thickness, such as a metal wire, bar or sheet.
2. Description of the Prior Art:
With its rapid growth, the electronic industry has incessantly been calling for still smaller and more precise machines and devices. They require metallic materials which are still smaller in thickness or diameter, and which are still better in quality. More specifically, they require thinner wires, sheets and foils formed from a material having a unidirectionally solidified structure, and which is free from any cavity or blowhole, and any grain boundary where impurities are likely to gather.
It is generally known that if a metallic material is subjected to cold working, such as rolling or drawing, it hardens and is eventually likely to fracture or crack at a primary crystal grain boundary which is formed in the casting process. Therefore, it is highly desirable to use a metallic material having a structure free from any such grain boundary in order to make a wire, sheet or foil which is extremely small in diameter or thickness.
There is known a method which is called zone melting, and which has primarily been developed for refining a metal. This method employs an elongated refractory vessel having a horizontal groove and called a boat. A metal ingot in the form of a bar is placed in the boat. Heat is applied from the outside of the boat by, for example, a resistance heater, gas burner or high-frequency induction coil to melt the ingot locally, and the melted portion of the metal is moved in one direction. This method not only refines the ingot, but can also produce an ingot having a unidirectionally solidified structure. The length of the ingot which this method can produce is, however, limited by the boat. It is incapable of making continuously an elongated ingot having a desired cross sectional shape and a unidirectionally solidified structure.
In order to produce an ingot in the form of a bar having a unidirectionally solidified structure, such as for a cast magnet, it has often been the case to place a metal bar vertically, heat it by a zone melting furnace having a resistance heater not contacting the bar, or a high-frequency induction coil to melt it locally, and move the melted portion of the bar upwardly or downwardly. This method does not employ any mold, but relies upon the surface tension of the molten metal to maintain the shape of the melted bar surface. The solidifed metal fails to retain the original smooth surface of the bar and has an uneven surface which has to be ground or polished before the bar can be used for making any product.